1. Field of the Invention
The invention regards a process and a device for determining a time interval, in particular with position sensors, according to the travel time principle, e.g. in magnetostrictive position sensors.
2. Description of the Related Art
In position sensors according to the travel time principle, a time interval between a starting impulse and a stop impulse is determined whose duration represents the distance of the position indicator, mostly a position magnet, from the measurement location.
Therefore, it is obvious that the precision of the position sensor among other things depends on the precision of the determination of the time interval.
In addition, in other technical applications a precise determination of a very short time interval is often required.
The patent document EP0885373 provides an application specific special circuit SARA {12}, comprising a high resolution time measurement unit additionally provided with a state machine with a solid state program. The state machine is capable of independently processing the base measuring cycle of a magnetostrictive position sensor in a continuous manner. For further tasks of the measuring process and for interface communication an additional microcontroller {16} is provided, connected with the SARA circuit via a parallel 8-bit data bus, comprising 12 conductors. A disadvantage of this setup is the low flexibility of the solid state program state machine and the additional effort for communication and division of the processing tasks between the state machine and the microcontroller. Also, the high number of housing connector pins prevents miniaturization. In order to achieve the high measuring resolution, precision and stability, as a reference element for the time measurement, a quartz is suggested. From this results a cost and design disadvantage, since quartzes are expensive in comparison to other resonators and require substantially more space. In order to improve the sensitivity of the setup against interferences (noise), an adaptive noise window has been suggested. This is intended to avoid an erroneous triggering of the time measurement through interference impulses, since the time measurement is only released within a predetermined position impulse expectation window. A disadvantage of the noise window is the high computation effort for the adaptive positioning of the noise window.
The Patent document EP01571425 suggests a magnetostrictive position sensor allocating a band pass filter to the impulse detector in order to make the analog impulse signal less sensitive to interferences. In order to allow the integration of the hardware circuits into a standard housing, a set up comprising four independent and double sided printed circuit boards, connected amongst each other via printed circuit board connectors, has been suggested. The housing itself comprises more than eight application specific formed parts. A disadvantage of this set up is not only the effort for the many housing components, but also the reliability of the high number of pin connectors for the internal wiring. The requirement of a band pass filter is in contrast to the goal of achieving a simple circuit.
The Patent document U.S. Pat. No. 5,640,109 suggests for improvement of the interference resistance of the impulse detector against electrical and mechanical interference impulses a measurement of the peak value and a corresponding threshold adaptation. The adaptation of the switching threshold through a digital/analog converter requires a high resolution in order to keep step errors through the threshold adaptation below the resolution of the sensor. Furthermore, the measurement of the peak amplitude requires an additional effort in the respective circuits. Alternative embodiments of analog threshold adaptation to avoid step errors significantly increase the hardware effort for comparators, operational amplifiers, hold elements, analog switches and consume more space on the electronics unit.
The Patent document U.S. Pat. No. 4,943,773 suggests introducing a second comparator in order to improve the interference resistance and detection precision in order to recognize a first switching threshold, unequal zero, with a subsequent detection during zero passage. Also, an additional effort and placement surface on the electronics group are required for implementing the additional comparator.